Method for Determining at least one State of a Plurality of Battery Cells, Computer Program, Battery and Motor Vehicle

ABSTRACT

The disclosure relates to a method for determining at least one state of a plurality of spatially combined battery cells connected to each other by circuitry. The state is determined by observing battery cells by means of at least one observer structure. A subset of a plurality of battery cells is observed and the state derived from the observation is determined for more battery cells than for the observed battery cells. The disclosure further relates to a battery comprising a battery management system, which is configured such that the method according to the disclosure can be carried out thereby. The disclosure also relates to a motor vehicle comprising a battery according to the disclosure.--

The present invention relates to a method for determining at least onestate of a plurality of spatially combined battery cells which areconnected to one another by circuitry.

Furthermore, the present invention relates to a computer program withwhich the method according to the invention can be implemented, and to abattery having at least one battery management system, wherein thebattery management system is embodied in such a way that the methodaccording to the invention can be implemented with it. Furthermore, theinvention relates to a motor vehicle having a battery according to theinvention.

A battery which comprises one or more galvanic battery cells serves asan electrochemical energy accumulator and energy converter. During thedischarging of the battery or of the respective battery cell, chemicalenergy stored in the battery is converted into electrical energy by anelectrochemical redox reaction. This electrical energy can therefore berequested by a user according to requirements.

In particular, in hybrid vehicles and electric vehicles, lithium-ionbatteries or nickel-metal-hydride batteries, which are composed of alarge number of electrochemical cells connected in series, are insertedinto what are referred to as battery packs. A battery management systemincluding a battery state detection means usually serves to monitorsafety and to ensure a service life which is as long as possible.

PRIOR ART

DE 199 59 019 A1 discloses a method for detecting a state of an energyaccumulator by means of which precise and reliable energy accumulatordiagnostics can be carried out using a model, a filter and a parameterestimator. By means of the estimation of parameters it is possible todetermine model parameters which come about owing to the aging of theenergy accumulator or owing to defects, and to continuously approximateand adjust the model, on which the estimations are essentially based, tothe actual state.

For the purpose of monitoring safety and to ensure a service life whichis as long as possible, it is also known to measure the voltage of eachindividual cell together with the battery current and the batterytemperature and to perform a state estimation in respect of the state ofcharge and/or the state of health.

In technically high-quality battery management systems control observerstructures are used. Such an observer structure is illustrated, forexample, in FIG. 1.

An observer is understood to be a system which determines and/or derivesstates by means of a model and by using known, defined input variablesand/or measurement variables. The magnitude of such states usuallycannot be measured owing to their complexity, or can be measured only atvery high cost.

In the model, the observer therefore models an actual controlled systemor a real system. It can comprise a controller which models themeasurable state variables. A known observer is what is referred to asthe Luenberger observer.

By using an observer or an observer structure as illustrated in FIG. 1,it is possible to precisely determine the states of the service life andthe performance of a battery pack at any time. The cell models which areused here correspond to theoretical figures or mathematical models. Theyhave numerous parameters for describing the capacity and the impedanceof the individual cell. Energy contents and performance of the cells andof the entire pack as well as service life predictions are calculatedfrom these parameters. These values can also be parameters of the cellmodels and/or of the observer structure.

The parameters themselves very frequently have multi- dimensionaldependencies on state variables such as, for example, temperature, stateof charge, current strength and similar variables. This thereforeresults in complex parameter spaces in which the parameters can beobserved. A dedicated observer is assigned to each cell.

For the purpose of correcting each cell model, adjustment has to becontinuously made between the cell model and the respective cell withrespect to the conditions or their variables, such as for exampletemperature, which are actually prevailing at or in the respectivebattery cell.

Conventional observer structure 20 is, as illustrated in FIG. 1,arranged parallel to a battery cell 10. The value of a current I_(Bat)which flows through the battery cell 10 is fed as information to theobserver structure 20. Likewise, the value of a temperature, measured ator in the battery cell 10, of the cell T_(cell) is fed to the observerstructure 20. The voltage V_(cell) which is measured at the battery cell10 is compared with the model voltage V_(Mod) determined in the cellmodel of the observer structure 20, and the value of the resultingdifference voltage 50 is fed again to the observer structure 20. Thestates of charge 30 and of health 40 are estimated by the observerstructure 20.

The illustrated observer structure 20 is not restricted to determiningthe state of charge 30 and the state of health 40 alone here but ratherfurther states can also be determined depending on requirements and as afunction of the calculation capacity. Likewise, the observer structure20 is not restricted to the inputting of the values of the currentstrength, temperature and voltage.

When the observer structure is applied for sensing states of batterycells, each individual cell is adjusted not only in its state of chargebut also in age-specific parameters such as what is referred to as the“state of health” (SOH), using the cell model and the control observerstructure.

It is disadvantageous here that this process entails enormouscomputational complexity and memory requirements, which also increaseproportionately with the number of cells. This is unfavorable withrespect to the scalability, usually desired by the automobile industry,of battery platform systems which are essential in performance whichdiffers with respect to the reduction of complexity and costs in themanufacture of battery systems of differing performance, and thecomputing power which is required as a result.

Disclosure of the Invention

According to the invention, a method is made available for determiningat least one state of a plurality of spatially combined battery cellswhich are connected to one another by circuitry, wherein thedetermination of the state is implemented by observing battery cells bymeans of at least one observer structure 20, and wherein a subset n_(x)of a totality of battery cells n_(tot) is observed and the state derivedfrom the observation is determined for more battery cells than for theobserved battery cells.

The model of the observer is here a cell model which is a mathematicalrepresentation of a battery cell which constitutes the actual controlledsystem.

Advantages of the Invention

As a result of the observation of just one subset n_(x) of the totalityof battery cells n_(tot), the computational outlay and memoryrequirements of the battery management system can be significantlyreduced.

In one refinement of the method according to the invention, the statecan be determined by means of estimation. Alternatively, a calculationcan also be performed. That is to say the observer structure estimatesor calculates the state or the states.

The method according to the invention for determining at least one stateof a plurality of spatially combined battery cells which are connectedto one another by circuitry can, in particular, be configured in such away that the respective state is determined both for the observedbattery cells and also for nonobserved battery cells.

The state is preferably determined for all the battery cells of therespective pack, that is to say for the battery cells which arespatially connected to one another by circuitry.

At least one state which can be determined is the state of charge or thestate of health. The method according to the invention is preferablyconfigured in such a way that both states can be determined at one ormore battery cells.

In a particularly advantageous embodiment of the method according to theinvention there is provision that the totality of all the battery cellsn_(tot) is divided into groups, wherein in each case one subset n_(i) ofbattery cells is assigned to a group, and wherein the observation takesplace groupwise in an alternating fashion. This means that the states ofthe groups of battery cells are determined in succession. It isappropriate here for the method to be sequenced in a loop. Thisalternating observation of a reduced number of cells instead of thesimultaneous observation of all the cells is an important aspect of theinvention. The groupwise observation permits the computationalcomplexity and memory requirements of the battery management system tobe significantly reduced, and at the same time the observation qualityis hardly decreased compared to observation of all the battery cellssince all the groups are observed in succession and the states which arederived or determined from the observation are referred to all thebattery cells.

In a preferred further refinement of the method there is provision thatthe observation is implemented by means of a cell model which has atleast one parameter from which at least one state of the battery cellcan be calculated.

The duration of the observation of the battery cells t_(B) preferablyhas a relationship of t_(B)/t_(obs)=1/5*10⁶ . . . 1/5*10⁵ with anobservation time period t_(obs) assumed for the totality of the batterycells.

A time constant τ_(obs) on which the cell model is based is alsoadvantageously significantly smaller than the observation time periodt_(obs). Therefore, for example, an observation time period t_(obs) often years or approximately 100 000 kilometers traveled can be assumedfor a battery which provides the driving power of a motor vehicle. Thiscorresponds to approximately 2000 operating hours. The time constantτ_(obs) can in this case be less than 1 hour, and the duration ofobservation of the battery cells t_(B) can be approximately 1 to 10minutes, preferably 1 to 5 minutes.

The knowledge of the basic method of functioning of the totality of thebattery cells and, in particular, of the uniform application of certaininfluences or physical variables to all the battery cells permits aconfiguration of the inventive method in which for at least oneparameter υ it is analyzed how large the variation σ of the parameterυ_(i) of the totality of the battery cells n_(tot) is, wherein in thecase of the variation σ not exceeding a previously defined limitingvalue σ_(max), an average change in the parameter Δυ_(k) is used for thedetermination of the respective state of all the battery cells of thetotality of the battery cells n_(tot).

This use of the average change in the parameter Δυ_(k) can take placehere by direct calculation thereof or in a weighting with a definedfactor.

As a result, a reduction in the observation quality can be avoided byutilizing the basic method of functioning of battery packs.

Furthermore, according to the invention, a computer program is madeavailable which, after it has been loaded into storage means of the dataprocessing device, permits said data processing device to carry out themethod according to the invention for determining at least one state ofa plurality of spatially combined battery cells which are connected toone another by circuitry.

The present invention is supplemented by a computer-readable storagemedium on which a program is stored which, after it has been loaded intostorage means of the data processing device, permits said dataprocessing device to carry out the method according to the invention; aswell as by a method in which the computer program is downloaded from anelectronic data network such as, for example, the Internet, to a dataprocessing device connected to the data network.

Furthermore, according to the invention, a battery, in particular alithium-ion battery or nickel-metal-hydride battery, is made available,wherein the battery comprises spatially combined battery cells which areconnected to one another by circuitry as well as at least one batterymanagement system, and can be connected or is connected to a drivesystem of a motor vehicle, wherein the battery management system isdesigned to implement the method according to the invention in order todetermine at least one state of a plurality of spatially combinedbattery cells which are connected to one another by circuitry.

Furthermore, according to the invention, a motor vehicle is madeavailable which comprises at least one battery according to theinvention.

By means of the method according to the invention it is possible toreduce the computational complexity and storage requirements of batterymanagement systems with respect to the adjustment of the parameters ofthe observed cells. In addition, the scalability of battery platformsystems and of the battery management systems connected thereto can beimproved with defined, limited hardware resources.

In addition to the advantages already mentioned, lower hardware costsand/or fewer hardware variants which are dependent on the number ofbattery cells are necessary. A relatively high observation quality isensured since no significant loss of information is found to occurduring the observation of subsets of battery cells, in particular duringthe alternating observation. The observer structure can be implementedby means of software and suitable processors and is accordinglycost-effective.

DRAWINGS

The invention will be explained in more detail with reference to thefollowing description and to the drawings, in which:

FIG. 1 shows a battery cell provided with a conventional observerstructure, and

FIG. 2 shows division of the battery cells and observer structures intogroups.

The conventional observer structure illustrated in FIG. 1 has alreadybeen described in the explanation of the prior art.

Embodiments of the Invention

In FIG. 2 the totality of all the battery cells which are to be sensedwith respect to at least one of their states is illustrated. The batterycells 10 and the observer structures 20 assigned thereto are dividedinto four groups 1, 2, 3 and 4.

The invention is, however, not restricted here to the embodimentaccording to FIG. 2, but instead, in contrast therewith, only aplurality of battery cells can also be combined in groups without therespectively assigned observer structure, wherein in each case a groupof at least one superordinate observer structure is observed. Thecomputer power of the observer structure should be correspondinglyadapted.

The method according to the invention is preferably carried out by analternating observation of, in each case, just a subset of the installedbattery cells 10, specifically the battery cells 10 assigned into, ineach case, one group 1, 2, 3 or 4. If the pack or the totality n_(tot)of all the battery cells 10 has n cells, a subset m =n/x canadvantageously be observed where n, x and m are advantageously integers.In the example in FIG. 2, n=24 cells in x=4 groups of m=6 cells each areillustrated. The observation of the individual groups alternates and thenumerical sequence can be selected as desired. Processing in a loop isparticularly suitable.

In order to fix the required hardware resources and therefore make itpossible to plan costs for the implementation of the method and keepthem low, in particular the number of simultaneously observed cells mduring the configuration of a battery pack can always be kept the sameor below a certain limiting value. As a result, only the number ofgroups x is scaled with the number of cells n in the pack or in thetotality n_(tot) of all the battery cells 10.

Since the cell parameters which are observed are aging-dependent andsince the aging of electrochemical cells usually takes place relativelyslowly, the observation time periods t_(obs) are correspondingly long.As long as it is ensured that the time constants τ_(obs) of theobservers are significantly shorter than the observation time periodst_(obs) (τ_(obs)<<τ_(obs)), the inventive method can be particularlyadvantageously applied. The duration of the observation of a group up tothe next changeover can be suitably selected but should likewise besignificantly shorter than the necessary observation time periods. It istherefore possible, for example, to assume an observation time periodt_(obs) of 10 years or approximately 100 000 kilometers traveled for abattery which provides the driving power of a motor vehicle. Thiscorresponds to approximately 2000 operating hours. The time constantτ_(obs) can be less than 1 hour in this case, and the duration of theobservation of the battery cells t_(B) can be approximately 1 to 10minutes, preferably 1 to 5 minutes.

Since all the cells in the observed totality of the battery cellsn_(tot) are preferably connected in series, they are loaded by the samebattery current. This also applies in a first approximation when thetotality of the battery cells n_(tot) is configured with a plurality ofparallel lines. As a result, the charge throughput is identical. Owingto the generally good thermal coupling between the battery cells 10 andtheir operation in spatial proximity, the thermal loading of theindividual battery cells 10 is as expected also similar. As a result,the two main causes of aging and therefore changes in parameters for theindividual battery cells 10 are virtually identical. This fact can beutilized to increase further the quality of the observation and ensurevirtually uninterrupted observation.

For this purpose, for a parameter υ of the cell model it is analyzed howlarge the variation σ of the parameters υ_(i) is of all the batterycells 10 in the pack or in the totality of all the battery cellsn_(tot). Provided that σ does not exceed a limiting value which is to bedefined, that is to say the battery cells 10 of the totality of all thebattery cells n_(tot) have similar behavior, the average change Δυ_(k)which is determined in the observed group can be transmitted, entirelyor weighted with a factor to be defined, to all the nonobserved batterycells 10.

As a result, in a battery pack or in the totality of all the batterycells n_(tot) it is possible, under certain, not improbable, conditions,for the observation quality of the cell parameters ideally to assume thesame value as in the case of observation of all the cells at the sametime, but with significantly reduced hardware expenditure.

1. A method for determining at least one state of a plurality ofspatially combined battery cells which are connected to one another bycircuitry, comprising: determining the at least one state by observing asubset of a totality of battery cells by means of at least one observerstructure; and determining the at least one state derived from theobservation for more battery cells than for the observed battery cells.2. The method for determining at least one state of a plurality ofspatially combined battery cells which are connected to one another bycircuitry as claimed in claim 1, wherein the at least one state isdetermined by means of estimation.
 3. The method for determining atleast one state of a plurality of spatially combined battery cells whichare connected to one another by circuitry as claimed in claim 1, furthercomprising: determining the at least one state both for the observedbattery cells and also for non-observed battery cells.
 4. The method fordetermining at least one state of a plurality of spatially combinedbattery cells which are connected to one another by circuitry as claimedin claim 1, wherein the at least one state is the state of charge or thestate of health.
 5. The method for determining at least one state of aplurality of spatially combined battery cells which are connected to oneanother by circuitry as claimed in claim 1, wherein: the totality of allthe battery cells is divided into groups, in each case one subset ofbattery cells is assigned to a group, and the observation takes placegroupwise in an alternating fashion.
 6. The method for determining atleast one state of a plurality of spatially combined battery cells whichare connected to one another by circuitry as claimed in claim 1, whereinthe duration of the observation of the battery cells has a relationshipof t_(B)/t_(obs)=1/5*10⁶ . . . 1/5*10⁵ with an observation time periodassumed for the totality of the battery cells.
 7. The method fordetermining at least one state of a plurality of spatially combinedbattery cells which are connected to one another by circuitry as claimedin claim 6, wherein: for at least one parameter it is analyzed how largethe variation of the parameters of the totality of the battery cells is,and in the case of the variation not exceeding a previously definedlimiting value, an average change in the parameter is used for thedetermination of the respective state of all the battery cells of thetotality of the battery cells.
 8. The method for determining at leastone state of a plurality of spatially combined battery cells which areconnected to one another by circuitry as claimed in claim 1, wherein: acomputer program which, after it has been loaded into storage means of adata processing device, permits said data processing device to carry outthe method for determining at least one state of a plurality ofspatially combined battery cells which are connected to one another bycircuitry.
 9. A battery, comprising: a plurality of spatially combinedbattery cells which are connected to one another by circuitry; and atleast one battery management system, wherein the battery is connectableto a drive system of a motor vehicle, wherein the battery is a lithiumion battery or a nickel metal hydride battery, wherein the batterymanagement system is configured to implement a method for determining atleast one state of the plurality of spatially combined battery cells,and wherein the method includes (i) determining the at least one stateby observing a subset of a totality of battery cells by means of atleast one observer structure, and (ii) determining the at least onestate derived from the observation for more battery cells than for theobserved battery cells.
 10. A motor vehicle comprising: at least onebattery including (i) a plurality of spatially combined battery cellswhich are connected to one another by circuitry and (ii) at least onebattery management system, wherein the battery is connected to a drivesystem of the motor vehicle, wherein the battery is a lithium ionbattery or a nickel metal hydride battery, wherein the batterymanagement system is configured to implement a method for determining atleast one state of the plurality of spatially combined battery cells,and wherein the method includes (i) determining the at least one stateby observing a subset of a totality of battery cells by means of atleast one observer structure, and (ii) determining the at least onestate derived from the observation for more battery cells than for theobserved battery cells.